ZHANG Ziqi; YAN Shuncheng; TAO Chenyu; YU Xuan; ZHANG Shaofeng; MA Xinwen

doi:10.7498/aps.74.20250008

Abstract

Molecular ions are widely distributed in the ionosphere of planetary atmospheres, and their fragmentations can generate different ions and neutral fragments. Studying the kinetic energy distribution and generation mechanism of the final products is helpful in understanding fundamental phenomena in astrophysics and plasma physics. In particular, ethane is an important molecule found in Titan and comet, and its fragmentation may be involved in the generation of complex hydrocarbons, as well as the atmospheric escape processes on Titan.In this paper, the experiment on ethane fragmentation by electron impact is carried out, focusing on the three-body fragmentation channel from $ {{\text{C}}_2}{\text{H}}_6^{2 + } $ to $ {\text{CH}}_3^ + /{\text{CH}}_2^ + /{\text{H}} $. The three-dimensional momenta of $ {\text{CH}}_3^ + $ and $ {\text{CH}}_2^ + $ ions are measured, and then the momentum of the H atom is reconstructed using momentum conservation law. Based on these analyses, the kinetic energy release (KER) spectrum and the fragmentation mechanisms are investigated.The time-of-flight (TOF) coincidence map of the ions shows two channels: channel (1) that represents the two-body dissociation generating $ {\text{CH}}_3^ + $/$ {\text{CH}}_3^ + $, and channel (2) that refers to the three-body dissociation generating $ {\text{CH}}_3^ + /{\text{CH}}_2^ + /{\text{H}} $. It is found that the neutral H from channel (2) has a wide kinetic energy distribution, ranging from 0 eV to more than 10 eV. This feature indicates that the dissociation of the C－H bond is from multiple electronic states. Since the escape threshold of H in Titan’s ionosphere is 0.02 eV, the vast majority of the H atoms produced in channel (2) can escape into outer space. In addition, the kinetic energy sum of $ {\text{CH}}_3^ + $ and $ {\text{CH}}_2^ + $ in channel (2) is found to be similar to the KER of channel (1), indicating that the C－H dissociation presents limited influence on the energy sum of the CH₂⁺ and $ {\text{CH}}_3^ + $.The corresponding fragmentation mechanism of channel (2) is also analyzed in this work. the overall KER spectrum is divided into three parts: 0–6 eV, 6–9 eV, and 9–11 eV, and the respective Dalitz plots and Newton diagrams are reconstructed under different KER conditions. In all Dalitz plots, there are a bright spot representing the concerted dissociation and a horizontal belt representing the sequential dissociation. The concerted dissociation is considered as the main mechanism, while the sequential dissociation plays a secondary role.The bright spot in the Dalitz plot shifts from the center to the left as the KER increases. This feature arises from the fact that the $ {\text{CH}}_2^ + $ lies between the H and the $ {\text{CH}}_3^ + $ in the concerted dissociation, and it feels the recoil both from H and from $ {\text{CH}}_3^ + $. Considering that the Coulomb potential from $ {\text{CH}}_3^ + $ is constant, the increase of the C－H dissociation energy will reduce the $ {\text{CH}}_2^ + $ kinetic energy. The belt in the Dalitz indicates that the sequential dissociation is a two-step process, with the first step being the dissociation of $ {{\text{C}}_2}{\text{H}}_6^{2 + } $ to generate H and metastable $ {{\text{C}}_2}{\text{H}}_5^{2 + } $, and the second step being the fragmentation of $ {{\text{C}}_2}{\text{H}}_5^{2 + } $ into $ {\text{CH}}_3^ + $ and $ {\text{CH}}_2^ + $.The Newton diagrams under different KER conditions are also reconstructed to give further evidence of the sequential dissociation from the metastable $ {{\text{C}}_2}{\text{H}}_5^{2 + } $, rather than from the metastable $ {\text{CH}}_3^ + $ or $ {\text{CH}}_4^ + $. In fact, for the former case, the center positions of the two half circles in the Newton diagram are correct. Oppositely, for the latter two cases, the center positions notably deviate from the expected values. This means the sequential dissociation from $ {{\text{C}}_2}{\text{H}}_5^{2 + } $ is dominant, which agrees excellently with the conclusion from the Dalitz plots.

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Authors and contacts

Corresponding author: YAN Shuncheng, yanshuncheng@impcas.ac.cn ; MA Xinwen, x.ma@impcas.ac.cn

Funds: Project supported by the National Key R&D Program of China (Grant No. 2022YFA1602500), the Strategic Priority Research Program (B) of Chinese Academy of Science (Grant No. XDB34020000), and the Foundation of “Young Scholars in Western China” of the Chinese Academy of Sciences.

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Vol. 74, Issue 6 - 2025-03-20

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